The present invention relates generally to discharge lamp ballasts and illumination fixtures containing discharge lamps powered by the ballasts. More particularly, the present invention relates to a discharge lamp ballast with feedback control during an electrode heating operation for providing optimal current flow through a discharge lamp.
Discharge lamp ballasts such as that shown for example in FIG. 9 are well known in the art for powering hot cathode type discharge lamps such as a high-pressure discharge lamp (or a high-intensity discharge lamp, also known as HID lamps). These ballasts generally include a DC-AC power converter and a control circuit for controlling the power converter.
The particular discharge lamp ballast 1 of FIG. 9 includes a direct-current (DC) power source E for converting alternating-current (AC) power, for example supplied from an AC power source AC such as a commercial power source, into DC power.
The DC power source E includes a diode bridge DB that full-wave rectifies an AC power input from the AC power source, a diode D0 having an anode coupled to a high-side output terminal of the diode bridge DB via an inductor L0 and a cathode coupled to ground via an output capacitor C0, a switching element Q0 with a first end coupled to a node between the inductor L0 and the diode D0 and a second end coupled to ground, and a driver circuit (not shown) for controlling the switching element Q0 to turn on/off and thereby maintaining a constant output voltage of the DC power source E (i.e., a voltage across the output capacitor C0). Specifically, the DC power source E is configured by coupling a commonly-known boost converter (i.e., a step-up chopper circuit) across output terminals of the diode bridge DB.
The conventional discharge lamp ballast 1 as shown in FIG. 9 further includes a power converter with four switching elements Q1 to Q4 arranged in a full bridge circuit configuration for converting a DC power input from the DC power source E into AC power. Field effect transistors (FETs or MOSFETS) can be used as the switching elements Q1 to Q4. A node between switching elements Q1 and Q2 is a first output terminal of the full bridge circuit and coupled to one end of a discharge lamp La (that is, one of the lamp electrodes) via a secondary winding of an auto-transformer AT. A tap in the auto transformer is further connected to ground via a first capacitor C1. A node between switching elements Q3 and Q4 is a second output terminal of the full bridge circuit and is coupled to the other electrode of the discharge lamp La via the inductor L1. A second capacitor C2 is further coupled on a first end between the switching elements Q1 and Q2 and on a second end between the inductor L1 and discharge lamp La. The auto-transformer At, the first capacitor C1, the second capacitor C2, and the inductor L1 collectively make up a resonant circuit coupled between the output terminals of the power converter (hereinafter referred to as “a load circuit”) together with the discharge lamp La.
The discharge lamp ballast 1 further includes a control circuit 2 for driving each of the switching elements Q1 to Q4. The control circuit 2 drives the switching elements Q1 to Q4 to be on or off so that a diagonally-positioned pair among the switching elements Q1 to Q4 (i.e., Q1 and Q4 or Q2 and Q3) can be turned on at the same time and a pair coupled with each other in series among the switching elements Q1 to Q4 (i.e., Q1 and Q2 or Q3 and Q4) can be alternately turned on or off. In this manner, DC power input from the DC power source E is converted into AC power, and a polarity reversal frequency of the AC power becomes a polarity reversal frequency generated by the above-mentioned on-off driving of the switching elements (hereinafter referred to as “an operating frequency”).
The control circuit 2 in a conventional example as shown in FIG. 9 carries out three control operations. First, a startup operation is conducted to ignite the discharge lamp La by relatively increasing an output voltage from the power converter. An electrode heating operation is conducted for relatively increasing a frequency of output power of the power converter to heat each electrode or filament of the discharge lamp La. Finally, a normal operation follows wherein an AC power is provided from the power converter for maintaining stable light output of the discharge lamp La.
Referring to FIG. 10, an example of operations by the control circuit 2 may be explained further. The first four waveforms of FIG. 10 show drive input signals to the respective switching elements Q1 to Q4, or more particularly, voltages applied between the gate and the source of each switching element. The respective switching elements Q1 to Q4 are turned on in periods when the drive signals are in an H level and turned off in periods when the drive signals are in an L level. Horizontal axes of the respective graphs of FIG. 10 represent time. When a power source is turned on, the control circuit 2 enters a startup operation to initiate discharge in the discharge lamp La. During a startup period P1 when the startup operation is carried out, the control circuit 2 sufficiently raises a voltage output Vla to the discharge lamp La (hereinafter referred to as “a lamp voltage”) to initiate discharge in the discharge lamp La by setting the operational frequency to approximately a resonant frequency of the load circuit consistent with a state where the discharge lamp La is extinguished (hereinafter referred to as “a low-load resonant frequency”), for example a few dozen kHz to a few hundreds kHz. The low-load resonant frequency is a resonant frequency (or 1/n multiplied by the resonant frequency where “n” is a whole number) of a resonant circuit made up of a primary winding of the auto-transformer AT (the portion coupled between the tap and the node between switching elements Q1 and Q2) and the first capacitor C1. When the lamp voltage Vla obtained by stepping up the resonant voltage generated in the startup period P1 with the auto-transformer AT is increased to a voltage required for ignition (i.e., the start of glow discharge), the discharge lamp La ignites and an output current starts to flow through the discharge lamp La. Specifically, the auto-transformer AT and the first capacitor C1 define a startup circuit.
After the startup period P1, the control circuit 2 shifts to an electrode heating period P2 wherein the electrode heating operation is carried out. In the example of FIG. 10, the operational frequency is maintained in the electrode heating period P2 at the same frequency as the operational frequency in the startup period P1.
After the electrode heating operation is completed, for example after a predetermined time, the control circuit 2 shifts to a normal period P3 when normal lamp operation is carried out. As the temperature in the discharge lamp La rises, the lamp voltage V gradually increases for a few minutes immediately after the shift to the normal period P3 and then stabilizes. The operational frequency fin the normal operation is, for example, a few dozen Hz to a few hundreds Hz. In the example of FIG. 10, the control circuit 2 in the normal period P3 controls output power to the discharge lamp La with a PWM control signal to turn on or off switching elements Q3 and Q4 at a duty ratio depending on a desired output power to the discharge lamp La and at a sufficiently higher frequency than the operational frequency fat which the switching elements Q1 and Q2 are turned on and off.
In the example of FIG. 10, because the same operating frequency is employed in the startup period P1 and in the electrode heating period P2, the amplitude of the lamp current Ila is lower than an amplitude It required to sufficiently heat an electrode of the discharge lamp La.
Referring now to FIG. 11, it has been proposed to decrease the operating frequency f when transitioning from the startup period P1 to the electrode heating period P2. Because the operating frequency f is in a range where amplitude |Ila| of the lamp current Ila decreases in a curved relationship with respect to increases in the operating frequency f as shown in FIG. 12, the control circuit 2 decreases the lamp voltage Vla to correspondingly increase the lamp current Ila in the electrode heating period P2 by adjusting the operating frequency f lower than the operating frequency f upon termination of the startup period P1. In this manner, the lamp current Ila in the electrode heating period P2 can be sufficiently increased, and the discharge in the discharge lamp La can be shifted from the glow discharge to the arc discharge and stabilized. In addition, each of the electrodes of the discharge lamp La is heated, and an asymmetry current condition caused by a temperature difference between the electrodes of the discharge lamp La is also decreased or eliminated after the electrode heating period P2.
As additionally shown in the example of FIG. 11, the control circuit 2 gradually increases the lamp voltage Vla during a first portion of the startup period P1 by gradually reducing the operating frequency f to approach a low-load resonant frequency.
Moreover, in the example of FIG. 11 the operating frequency f is further decreased from a first portion to a second portion of the electrode heating period P2. Both of the two operating frequency f values in the two displayed portions of the electrode heating period P2 are predetermined values.
According to a conventionally known discharge lamp ballast as described above, the discharge in the discharge lamp La is shifted from glow discharge to arc discharge in an electrode heating operation, thereby stabilizing the discharge after transition to a normal operation in comparison to a case where the electrode heating operation is not carried out, and preventing the lamp from being suddenly and undesirably extinguished.
The impedance of the load circuit varies due to characteristics of circuit components and of the discharge lamp La and further due to an ambient temperature. Accordingly, when an operating frequency f value for the electrode heating operation is predetermined, the lamp current may be insufficient in the electrode heating operation, and accordingly the light output from the discharge lamp La in the subsequent normal operation is not stabilized. Conversely, an excessive lamp current may be provided during the electrode heating operation and electric stresses may therefore be applied to the circuit components and the discharge lamp La.